Mid-IR frequency comb and quantum-cascade laser system improves precision spectroscopy

When designing spectrometers, improving the range of available frequencies typically comes at the cost of decreasing the resolution. This can mean losing access to molecular information, like spectral features broadened by the Doppler effect. Trifluoromethane, an important chemical in the emerging field of cold stable molecules, for example, is hard to analyze with traditional spectroscopy methods because of its very narrowly packed absorption spectrum.

Researchers designed and tested a new mid-infrared (IR) spectrometer that can perform gigahertz-wide scans and still features a frequency resolution of only about 160 kilohertz (kHz). The instrument is based on a quantum-cascade laser (QCL) and a widely tunable mid-IR frequency comb and offers a high signal-to-noise ratio. The emission frequency of the QCL is offset-locked to one of the frequency comb’s teeth, proving the spectral precision to perform sub-Doppler measurements with high accuracy and sensitivity in direct-absorption and modulation-spectroscopy approaches.

In their APL Photonics paper, the authors describe the design of the mid-IR spectrometer and detail a system performance test with trifluoromethane (CHF₃) gas. The team says its new mid-IR spectrometer is an ideal tool to analyze spectroscopic parameters of molecules like CHF₃ because its novel absolute-calibrated scanning approach allows for the analysis of sub-Doppler spectral features.

Using the spectrometer, the team conducted three different types of Doppler-free measurements on CHF₃ gas at room temperature, in both direct spectroscopy and modulation spectroscopy configurations. Results from all three showed that the mid-IR spectrometer can perform absolute-frequency calibrated gigahertz-wide scans with a resolution of 160 kHz and accuracy of 10 kHz.

Moreover, the authors report a signal-to-noise ratio for their 10-second acquisition time that is the best achieved for a direct-absorption measurement without exploiting an optical enhancement cavity. They plan to use the system for accurate sub-Doppler line-profile investigations as well as collisional-effects and hyperfine-structure determinations in rarefied gases.

Source: “Versatile mid-infrared frequency-comb referenced sub-Doppler spectrometer,” by A. Gambetta, E. Vicentini, N. Coluccelli, Y. Wang, T. T. Fernandez, P. Maddaloni, P. De Natale, A. Castrillo, L. Gianfrani, P. Laporta, and G. Galzerano et al., APL Photonics (2018). The article can be accessed at https://doi.org/10.1063/1.5025135 .